You
1. You didn't follow up after the first sale. After the sale you
could have introduced your other producton the thank e-mail.
2. You didn't ship the product in the about of time you stated.
If they needed it in a hurry and youdidn't provide, they won't
rely on you again.
3. Your product didn't do as promised. If your product didn't
accomplish their desired goal they're not going to think your
second product will either.
4. Your customer couldn't get a hold of you in time when they
had a "after question" sale. You could have added extra lines
of communication.
5. Your customer doesn't want to revisit your website because
it didn't offer much. You could have offered more original content
or freebies.
6. Your competition is offering free shipping with their
product. You should have been more aware of how they are
targeting your customers.
7. Your customer forgot your web site address.You should have
given your customers your website information in your product
package.
8. Your customer service couldn't solve a problem they had with
your product. Your customer service should be trained to handle
most problems.
9. You didn't up-sell when they were already in the buying mood.
You can always try to sell your other product when they're ready
to buy your first one.
10. Your competition offers a stronger money back guarantee.
You must always be thinking of better ways to remove the risk
from your customers.